1. Field of the Invention
This invention relates to an improved apparatus for testing the linearity of a circuit such as audio amplifiers. The principles of this invention also have application to measurement of nonlinear distortions introduced in a signal transmission system.
2. Description of the Prior Art
In the past, to test a circuit for linearity, a sine wave test signal has been applied to a circuit under test. The output of the circuit under test, altered in accordance with the linearity of the circuit under test, is applied to a notch filter for filtering out the fundamental frequency of the input test signal. As a result, any variation in linearity of the circuit under test will cause harmonic distortion, i.e., higher harmonics in the output of the notch filter which is measured and thus provides a linearity indication.
Such an approach, generally known as the "total harmonic distortion measurement", is unsatisfactory to test linearity in circuits such as high-quality audio amplifiers in which a high degree of linearity is desired. With distortions in the input sine waveform, the harmonic distortion introduced in the circuit under test will vary accordingly causing a false linearity test. These variations in harmonic distortion appear to the test circuit as a difference in linearity, which in fact, is not the case. Furthermore, the background noise originating within the test circuit, especially filters provided at the output side thereof is superimposed upon the test signal so as to falsify the results of the measurements made. Because of these limitations, the measurement resolution in terms of the lowest measurable distortion is in the range of 0.001 to 0.01%. Another drawback of this method is its inability to provide a direct indication of the form of nonlinear distortions. In addition, this total harmonic distortion measurement is time-consuming.
It has long been recognized that the total harmonic distortion method generally does not give good correlation with subjective assessment of sound quality. In an effort to provide improved subjective agreement, R. A. Belcher proposed a new technique for measuring nonlinearity distortion, known as the double comb filter method (see "A New Distortion Measurement" by R. A. Belcher, Wireless World, May 1978, pp. 36-41). This technique uses two pseudo-random noise signals combined to provide a test signal with anharmonic components. Based on the recognition that a pseudo-random noise signal has itself a comb-like spectrum, two comb filters are employed having different comb characteristics, which are connected in cascade to reject the test signal from the output from the circuit under test. The distortion signal appearing at the output of the cascaded comb filters is measured as an indication of the linearity of the circuit. This comb filter method has proved to be effective, but suffers from the disadvantage that the circuitry required is considerably complex.
Another way to measure nonlinearity distortion is disclosed in an article entitled "Transient Nonlinear Distortion in Audio Equipment and Method of Measuring Same" by Yoshimutsu Hirata ("Japanese Acoustical Society Transaction" Vol. 1-2-16, October 1977, pp. 79-80). According to this method the test signal employed has no DC component, comprising a positive pulse and a negative pulse, in combination, the amplitudes of which are different from each other. In accordance with the teaching of that article, the test signal is generated by a clock-operated counter and a logic circuitry whose output is applied to a circuit under test after digital to analog conversion. The level of the resulting DC component or an increase of a certain low frequency component of the test signal as altered by the circuit under test indicates the linearity of the circuit. This method provides distortion figures that correlate much better than the total harmonic distortion measurement with subjective estimates of sound quality. However, it is desirable to provide a straightforward and inexpensive apparatus for testing a circuit for linearity.